Mass spectrometers are used extensively in the scientific community to measure and analyze the chemical compositions of substances. In general, a mass spectrometer is made up of a source of ions that are used to ionize neutral atoms or molecules from a solid, liquid or gaseous substance, a mass analyzer that separates the ions in space or time according to their mass or their mass-per-charge ratio, and a detector. Several variations of mass spectrometers are available, such as magnetic sector mass spectrometers, quadrupole mass spectrometers, and time-of-flight mass spectrometers.
The magnetic sector mass spectrometer uses a magnetic field or combined magnetic and electrostatic fields to measure the ion mass-per-charge ratio. In one type of magnetic sector geometry, {see A. O. Nier, Review of Scientific Instruments, Vol. 18 (1947) p. 398; L. Holmlid, International Journal of Mass Spectrometry and Ion Physics, Vol. 17 (1975) p. 403} only one mass-per-charge species is detected at any one time, so the magnetic field strength and, if present, the electric field strength must be varied in order to obtain a mass spectrum comprising multiple mass-per-charge species. Major limitations on this type of mass spectrometer are the high mass of the magnet and the time that is required to scan the entire mass range one mass at a time.
Another type of magnetic sector mass spectrometer creates a monoenergetic beam of ions, which are spatially dispersed according to mass-per-charge ratio, and which are focused onto an imaging plate {e.g., J. Mattauch and R. Herzog, Zeitschrift fur Physik, Vol. 89 (1934) p. 786}. While this type of spectrometer can detect multiple mass-per-charge species can be detected simultaneously, the poor spatial resolution it provides limits its use to a narrow mass range.
Quadrupole mass spectrometers utilize a mass filter having dynamic electric fields between four electrodes {e.g., Quadrupole Mass Spectrometer and its Applications, ed. Peter H. Dawson (American Institute of Physics, New York, 1995)}. These fields are tailored to allow only one mass-per-charge ion to pass through the filter at a time. Major limitations of quadrupole mass spectrometers are the high mass of mass of the required magnet and the time required to scan the entire mass range one mass at a time.
Time-of-flight mass spectrometers (TOFMS) can detect ions over a wide mass range simultaneously {see W. C. Wiley and I. H. McLaren, Rev. Sci. Instrum., Vol. 26 (1955) p. 1150; M. Guilhaus, J. Mass Spectrom., Vol. 30 (1995) p. 1519}. Mass spectra are derived by measuring the times for individual ions to traverse a known distance through an electrostatic field free region. In general, the mass of an ion is derived in TOFMS by measurement or knowledge of the energy, E, of an ion, measurement of the time, t1, that an ion passes a fixed point in space, P1, and measurement of the later time, t2, that the ion passes a second point, P2, in space located a distance, d, from P1. Using an ion beam of known energy-per-charge E/q, the time-of-flight (TOF) of the ion is tTOF=t2xe2x88x92t1, and by the ion speed is v=d/tTOF. Since E=0.5 mv2, the ion mass-per-charge m/q is represented by the following equation:                               m          q                =                                            2              ⁢                              Et                TOF                2                                                    qd              2                                .                            10      
The mass-per-charge resolution, commonly referred to as the mass resolving power of a mass spectrometer, is defined as:                                                         Δ              ⁢                              xe2x80x83                            ⁢                              m                /                q                                                    m              /              q                                =                                                    Δ                ⁢                                  xe2x80x83                                ⁢                E                            E                        +                          2              ⁢                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                                      t                    TOF                                                                    t                  TOF                                                      +                          2              ⁢                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  d                                d                                                    ,                    11      
where xcex94E, xcex94tTOF, and xcex94d are the uncertainties in the knowledge or measurement of the ion""s energy, E, time-of-flight, tTOF, and distance of travel, d, respectively, in conventional time-of-flight spectrometers.
In a gated TOFMS in which a narrow bunch of ions is periodically injected into the drift region, uncertainty in tTOF may result, for example, from ambiguity in the exact time that an ion entered the drift region due to the finite time, xcex94t1, that the gate is xe2x80x9copen,xe2x80x9d i.e. xcex94t1≈xcex94tTOF. The ratio of xcex94tTOF/tTOF can be minimized by decreasing xcex94tTOF, for example, by decreasing the time the gate is xe2x80x9copen.xe2x80x9d This ratio can also be minimized by increasing tTOF, for example, by increasing the distance, d, that an ion travels in the drift region. Often, a reflectron device is used to increase the distance of travel without increasing the physical size of the drift region.
Uncertainty in the distance of travel, d, can arise if the ion beam has a slight angular divergence so that ions travel slightly different paths, and, therefore, slightly different distances to the detector. The ratio of xcex94d/d can be minimized by employing a long drift region, a small detector, and a highly collimated ion beam.
The uncertainty in the ion energy, E, may result from the initial spread of energies, xcex94E, of ions emitted from the ion source. Therefore, ions are typically accelerated to an energy, E, that is much greater than xcex94E.
A further limitation of conventional mass spectrometry lies in the fact that the source of ions is a separate component from the time-of-flight section of a spectrometer, and it requires significant resources. First, most ion sources are inherently inefficient, so that few atoms or molecules of a gaseous sample are ionized, thereby requiring a large volume of sample and, in order to maintain a proper vacuum, a large vacuum pumping capacity. Second, the ion source typically generates a continuous ion beam that is gated periodically, creating an inefficient condition in which sample material and electrical energy are wasted during the time the gate is xe2x80x9cclosed.xe2x80x9d Third, ions have to be transported from the ion source to the time-of-flight section, requiring, among other things, electrostatic acceleration, steering and focusing. Fourth, typical ion sources introduce a significant spread in energy of the ions so that the ions must be substantially accelerated to minimize the effect of this energy spread on the mass resolving power. Finally, having an ion source separate from the drift region creates an apparatus having large mass and volume.
Still another problem with conventional time-of-flight mass spectrometers is that ions must be localized in space at time t1 in order to minimize xcex94d and therefore, minimize the mass resolving power. Typically, time t1 corresponds to the time that the ion is located at the entrance to the drift region.
In addition to these limitations that impact the mass resolving power of prior art TOFMS, an important further limitation is the intrinsically low duty cycle associated with the gating process in which a group of ions briefly is admitted at time t1 into the drift region. The time that must elapse before another group of ions can be admitted is determined by the time required for the heaviest, and therefore the slowest, ion, admitted at time t1, to reach the detector. This generally long period is necessary to prevent overlap of mass spectra, which are accumulated each gating period. Although sophisticated techniques have been developed in attempts to improve this limitation, the duty cycle of prior art TOFMS is still much less than unity.
One method of attempting overcome this limitation in TOFMS utilizes a thin foil located at the entrance to the drift region. A sample ion having sufficient energy to traverse the foil will continue to the xe2x80x9cstopxe2x80x9d detector. Secondary electrons generated by the interaction of the sample ion with the foil are detected, and provide a measurement of the time t1 that the ion entered the drift region. However, this method is not without its own limitations. These limitations include the requirement that the incident ion have sufficient energy to transit the foil, the energy degradation of the sample ion due to interaction with the foil, and the angular scattering of the sample ion due also to its interaction with the foil.
The present invention solves these problems of the prior art by continuously ionizing sample atoms or molecules that are initially at rest inside a drift region having an electric field. The secondary electron that is created by the ionization event is detected to provide the time t1, and the detection of the ion at a xe2x80x9cstopxe2x80x9d detector provides the stop time t2.
In order to achieve the objects and purposes of the present invention, and in accordance with its objectives, a time-of-flight ion mass spectrometer comprises an evacuated enclosure, with means for generating an electric field located in the evacuated enclosure, and means for injecting a sample material into the electric field. A source of continuous ionizing radiation injects ionizing radiation into the electric field to ionize atoms or molecules of the sample material, and timing means for determining time elapsed between arrival of a secondary electron having a particular energy out of the ionized atoms or molecules at a first predetermined location and arrival of a sample ion having a particular energy out of the ionized atoms or molecules at a second predetermined location.